von Willebrand factor (VWF) is a key component of the blood coagulation system. Deficiencies of VWF result in the most common inherited bleeding disorder in humans, von Willebrand disease (VWD). In contrast, elevated levels of VWF and the associated protein, factor VIII, are a major risk factor for thrombosis. This is an application for competing renewal of a research program focused on the molecular genetics of VWF. Considerable progress was made over the last funding cycle, including the identification of multiple genetic factors regulating the plasma level of VWF in laboratory mice and in humans. In addition, the genes for 2 related disorders of VWF and FVIII were identified: ADAMTS13 as the cause of familial thrombotic thrombocytopenic purpura (TTP) and MCFD2/LMAN1 as the cause of combined deficiency for factors V and VIII. The current proposal will continue and extend the body of work from the previous funding cycle. The 8 genetic loci that were previously identified to contribute to variability in plasma VWF levels among inbred mouse strains (Mvwf1-8) will be further characterized at the molecular level. Building on initial preliminary studies, direct genetic analysis for human modifiers of VWF and other hemostatic factors will be conducted using a cohort of 1182 healthy young subjects collected at the University of Michigan. In addition, a new approach will be undertaken to extend this genetic strategy to identify potential genetic modifiers of VWF levels in a zebrafish model system. The genetic power of the mouse and zebrafish systems may facilitate the identification of the critical genes modifying plasma VWF levels in humans, as well as yielding information about fundamental VWF biology that could lay the ground work for novel approaches to diagnosis and treatment in the future. Finally, the application of new high throughput sequencing technologies to a modified phage display strategy will be used to finely map functional domains within the VWF protein and to select for novel recombinant proteins with potential therapeutic utility.